Inclusions present in steel have an influence on various mechanical properties. For example, in the case where a steel sheet is blanked or punched out by press working or cut, there has been a technique of causing inclusions to finely disperse in order to improve blanking and cutting properties by means of the inclusions serving as initiation points of fracture.
On the other hand, it has been known also that the inclusions in steel adversely affect mechanical properties of the steel depending on chemical compositions, shapes and/or sizes thereof. With regard to a steel material which is required to have good fatigue strength, for example, there is a problem of fracture due to inclusions as an initiation point is problematic in use in which fatigue strength is required. A size of inclusions has a great influence on such fatigue fracture, and control of the inclusions present an important problem.
As a method of controlling inclusions in higher grade materials used for special applications such as automobile components, tool steel, structural steel, etc., two melting steps have been commonly performed, which consist of, for example, a first step of melting in an arc furnace (herein below referred to as AF) or vacuum induction melting (herein below referred to as VIM), and a second step of electroslag remelting (herein below referred to as ESR) or vacuum arc remelting (herein below referred to as VAR).
Steel produced through such two melting steps with use of VAR or ESR has an advantage that it is homogeneous (less component segregation) and contains a small amount of inclusions.
Maraging steel is a typical steel which is strictly required to overcome the problem of fatigue fracture due to inclusions.
Since the maraging steel has high toughness and high strength, it is used for structural members, on which a reiterative stress is exerted, and for important members which are required to have a high fatigue strength. However, it has been widely known that when a large-sized non-metallic inclusions are contained in such a member, fatigue fracture is liable to occur because the inclusions serve as initiation points of fracture. Thus, it is needed to finely disperse the non-metallic inclusions especially in order to prevent occurrence of high-cycle fatigue fracture.
In order to overcome the inclusion problem, there have been proposed various ideas of making inclusions fine. Such proposals can be seen from, for example, JP-A-11-293407 and JP-A-2003-183765 which were filed by the present applicant.